Afghan President Hamid Karzai doesn't have enough support to avoid a run-off vote in the upcoming presidential election, a new U.S. government-funded poll suggests.

Released earlier this week, the poll gave Karzai 36 per cent of the vote, followed by his main opponent, Abdullah Abdullah, a former foreign minister in the Afghan government, with 20 per cent.

Ramazan Bashardost was third with seven per cent and Ashraf Ghani had three per cent, according to the survey from Washington-based firm Glevum.

A candidate needs 50 per cent of the Aug. 20 vote in order to avoid going to a second, run-off election.

The poll also suggests close to 20 per cent of Afghans are undecided or wouldn't answer the survey questions.

Karzai, who is considered the front-runner, won the last presidential vote in 2004 with about 55 per cent of the vote.

According to the United Nations, there are 41 presidential candidates and more than 3,000 people running for provincial council seats.

In an effort to clamp down on Taliban efforts to disrupt the upcoming vote, U.S. marines on Wednesday stormed the Taliban-held town of Dahaneh in Helmand province in a move to gain control of the region.

U.S. President Barack Obama has made the war in Afghanistan his country's main overseas military priority, promising 21,000 more soldiers by the end of the year.

More than 2,800 Canadians are serving in Afghanistan, mostly in the violent Kandahar region.

The poll was a face-to-face survey of 3,556 Afghans aged 18 or older between July 8 and July 17. The margin of error is plus or minus 1.64, 19 times out of 20.